Brake Master Cylinder Bleeding

[JEFFTATE]

Please help !

I'm having trouble bleeding the new stock replacement brake master cylinder on my '69 Camaro.
I got one from Year One a while back.
It's the kind with bleeder screws on the master cylinder itself.
It's been sitting on a shelf for a while.
It bench bleeds fine, but when I install it on the car , I continue to get air from the bleeder fitting at the calipers.
(I've got a clear bleeder hose in a bottle of fluid.)
All the fittings are tight.
There's no leaks.
The reservoirs are full.
I've put 2 whole big bottles of brake fluid through , and I continue to get air out of the front lines.
I even put Teflon tape on the bleeder screw threads to keep from sucking air back past the threads.
I've tried a vacuum bleeder also. Same result.

I thought that maybe the master cylinder had a bad seal inside it.
So I got another one from a parts store because I didn't have time to wait for a replacement from Year One.
The one from the parts store does the same thing.
I'm trying to get the car ready to go to the Motor State Challenge in 4 days !!

The directions that came in the new master cylinder suggested that
it may be necessary to lower the front of the car to get the master cylinder level. To help prevent trapped air inside it.
But why ???
I bench bled it.
There's no air in it.
Where's all the air coming from ???
Could it be sucking air in from somewhere ??
Like the rear of the master cylinder ???



I've bled and flushed a hundred cars' brake systems as part of routine maintenance. ( I used to be a tech )
And once I'd gotten the air out of the master cylinder , the remaining air in the lines just pushed right through and came out at the caliper bleeder screws .
So what's wrong here ???

Why is a '69 Camaro different from say .....a Volvo , or a BMW ???

Please help !!
I've got a deadline !!
I've gotta' get this car ready to go to Michigan in 4 days.
My car is on jackstands and my hands are dirty !!!